A subnetwork, commonly referred to as a subnet, is a logical subdivision of a Layer-3 network. Network ports of nodes within a given subnet share the same Layer-3 network address prefix. For example, in Internet Protocol (IP) networks, the ports in each subnet share the same most-significant bit-group in their IP address, so that the IP address is logically divided into two fields: a network or routing prefix, and the rest field or host identifier. Similarly, in InfiniBand™ (IB) networks, each subnet is uniquely identified with a subnet identifier known as the Subnet Prefix. For each port in the subnet, this prefix is combined with a respective Globally-Unique Identifier (GUID) to give the IB Layer-3 address of the port, known as the Global Identifier (GID).
Typically, the logical subdivision of a Layer-3 network into subnets reflects the underlying physical division of the network into Layer-2 local area networks. The subnets are connected to one another by routers, which forward packets on the basis of their Layer-3 (IP or GID) destination addresses, while within a given subnet packets are forwarded among ports by Layer-2 switches or bridges. These Layer-2 devices operate in accordance with the applicable Layer-2 protocol and forward packets within the subnet according to the Layer-2 destination address, such as the Ethernet™ medium access control (MAC) address or the IB link-layer Local Identifier (LID). In general, Layer-2 addresses in a given subnet are recognized only within that subnet, and routers will swap the Layer-2 address information of packets that they forward from one subnet to another.
In IB networks, a Subnet Manager (SM) in each subnet assigns an LID to each physical port of each host within the given subnet. A subnet administration (SA) function provides nodes with information gathered by the SM, including communication of the LID information to a Subnet Management Agent (SMA) in each node of the subnet. For simplicity and clarity in the description that follows, all of these subnet management and administration functions will be assumed to be carried out by the SM. Layer-2 switches within the subnet are configured by the SM to forward packets among the ports on the basis of the destination LID (D-LID) in the packet header. The SM is typically implemented as a software process running on a suitable computing platform in one of the nodes in the subnet, such as a host computer, switch or appliance.
Routing protocols are used to distribute routing information among routers, so as to enable each router to determine the port through which it should forward a packet having any given Layer-3 destination address. In IP networks, the routing information is developed and distributed by and among the routers themselves. A number of routing protocols are commonly used to exchange routing information among IP routers, such as Open Shortest Path First (OSPF) and the Border Gateway Protocol (BGP).